Position encoders are used in a variety of applications to provide feedback identifying the position of a moveable member. Such moveable members are often used to identify the position and/or velocity of rotatable members, such as a steering wheel for a vehicle, a rotor shaft in a brushless motor, or other structures in which the position and/or velocity of a rotating member relative to a stationary member needs to be tracked or identified. In addition, encoders may also be implemented in applications for tracking linear movement of moveable members.
One commonly used encoder comprises a magnet structure including a sense magnet that is moveable relative to a sensor. In particular, in a known sense magnet assembly, a continuous annular ring of magnet material is assembled to a structure supported on a rotatable shaft. The magnet material is magnetized to provide a plurality of circumferential patterns defined at different diametric locations within the magnet material. For example, the magnet material may be magnetized to define a dual track sense magnet including an inner magnetic track and an outer magnetic track located radially outwardly from the inner track. The inner track may comprise a first number of sectors defined by a plurality of alternating magnetic poles, and the outer track may comprise a second number of sectors defined by a plurality of alternating magnetic poles, where the second number of sectors is larger than the first number of sectors. A Hall device is located adjacent to each of the tracks to detect transitions between the alternating poles defining the sectors. That is, the Hall devices detect switches between the alternating magnetic poles within a switching zone around the zero Gauss crossing of each transition of the magnet material's magnetic flux pattern.
As shown in FIG. 4B, for example, there is a transition slope in the flux density proceeding from one magnetic pole to an adjacent magnetic pole. Further, the switching zone results from a characteristic of the Hall devices in that a Hall device does not change state or detect a switch in flux direction exactly at zero Gauss, but instead has some difference or hysteresis between the operating and switching points of the Hall device. The accuracy of the encoder is directly affected by the width of the switching zones. Hence, it is desirable for the slope of the flux density to be a steep as possible such that the width of each switching zone is minimized.
The width of the switching zone is a function of several variables including the characteristics of the magnet material compound itself, the magnetization process used to define the poles, the characteristics of the particular Hall device(s) used, the proximity of the Hall device(s) to the magnet material, as well as other factors. In addition, in a multi-track sense magnet in which the magnetic tracks are located adjacent to each other, magnetic “cross-talk” typically occurs in which the magnetic field of a first magnetic track alters the magnetic field of an adjacent second magnetic track and adversely affects the switching zone detected by the Hall device(s), with an associated decrease in detection accuracy.